The earliest Ornithischians include the poorly known Pisanosaurus form South America, and genera such as Heterodontosaurus and Scutellosaurus.

Unlike the Saurichians, they had no teeth in the front of their jaws. Instead, there was a bony structure. In sone ornithischians, this structure developed into a beak very much like that of a bird. The beak was very useful in cutting through tough leaves and woody stems that formed the animal's diet. It also connected the two halves of the lower jaws and enabled them to transmit and absorb force during chewing. the ornithischians evolved later than the saurichians.

Stegosaurs were bulky, four-legged, had various kinds of body armor, and resembled gigantic porcupines. Some stegosaurs had two rows of spines along their back, while others had two rows of stiff, vertical plates. The tail was armed with pairs of bony spikes. Some believe that the plates and spikes may have helped protect the animals from enemies. These plates may have helped control the animal's body temperature. According to this theory, overheated blood was pumped through the thin plates and returned to the rest of the body. Air moving around the stegosaur's back would have cooled the blood as it flowed through the plates. The plates could also have warmed the blood by absorbing heat from the sun.

The Stegosaurus bore these double row of triangular bony plates along its back. Rather small, weak teeth may have limited their existence to the Jurassic period. Because of the difference in leg length (the front legs were shorter than the back legs), stegosaurs walked with their head close to the ground, making them look bent over.

During the Cretaceous Period, ankylosaurs replaced stegosaurs. Most kinds of ankylosaurs grew 15 to 30 feet (5 to 9 meters) long and had a skull 2.5 feet (80 centimeters) long. Looking like giant horned toads, they were encased in a mosaic-work shield of bony plates and spikes that covered the back and the head. Some had a bony plate in each eyelid. Many of the plates had ridges or spikes. In some ankylosaurs, large spikes also grew at the shoulders or at the back of the head. Some kinds of ankylosaurs had a large mass of bone at the end of the tail. This bone could be used as a powerful club against enemies.

The bipedal Ornithopods, also called duck-billed dinosaurs, became the most abundant plant-eating dinosaurs during the Cretaceous period, probably migrating in herds of thousands. They ranged in size from 2 m (6 ft) long to 10 m (32 ft) long and weighed from 15 kg (33 lb) to 4 metric tons.

They appeared at the beginning of the Age of Reptiles as small small-tusked animals (Heterodontosaurus) and persisted through the era, giving rise to the Iguanodon and hadrosaurs.

All were adept bipeds, but they were also able to assume a four-legged stance and gait. Adult duckbill dinosaurs, though, suffered many crushing fractures of the spool-shaped vertebrae in their tails that sometimes healed at unusual angles. It's probably because they're living in herds and they're just stepping on each others' tails.

The most interesting feature of the ornithopods is the specialized dentition many had for crushing and grinding plant material. this dental apparatus was most highlt developed in the duck-billed dinosaurs (Corythosaurus, Anatosaurus).

The Hadrosaurs have been given the nickname "duck bills" because of their mysteriously shaped mouths. They looked like flat, toothless beaks. Their grining dental batteries composed of hundreds of tightly compacted teeth, evidence of high food-volume intake, related perhaps to high metabolism. Even more interesting was the rest of the head. It had a crest, which was a long nose-bone, sticking out on top of the head. Several different shapes and sizes of these crests existed. Samples of Hadrosaurs have even come with intact skin, so we know almost exactly how they looked like. Some hadrosaurs were 9 feet (2.7 meters) tall at the hips and more than 30 feet (9 meters) long.

Iguanodons weighed as much as 4.5 metric tons, was 7.5 m (25 ft) long, and stood up to 4.5 m (15 ft) tall on its hind legs. The long, flat head ended in a horny beak, and its jaws contained teeth somewhat like those of an iguana. They had a bony spike on the thumb of each forelimb.

The quadrupedal "horned reptile," ceratopsians, including Triceratops, usually had horns over the nose and eyes and a saddle-shaped bony frill extending from the skull over the neck. They may have lived in herds.

Artists have long drawn horned dinosaurs like Triceratops using their horns to defend against attacks, but a study of horned dinosaurs turned up few injuries to the horns themselves, suggesting they were intended less as weapons than elaborate decorations.

Most were four-legged and stocky, like the rhinoceroses of today, and all were plant eaters. Many fossils of ceratopsians found in the same area suggest that they roamed in herds, and faced threatening meat eaters in packs. As the ceratopsians evolved, their head gear gradually became more spectacular.

Protoceratops ("first horned face") was a small, plant-eating dinosaur living in the Mongolia area about 97 million years ago to about 65 million years ago. The forerunner of the ceratopsians, Protoceratops grew to a length of about 1.8 m (about 6 ft), weighed about 347 pounds (180 kg) and walked on four legs. It would have been hip-high to a human adult. It had a short tail and a small bony neck frill at the back of its skull where its strong jaw muscles attached. Its facial horns were only just beginning to evolve, perhaps just as a bump on the nose and ridges over the eyes. Holes in the frill made the skull light so that the animal could make fast, defensive movements. Protoceratops held its head low for grazing, using its parrotlike beak to snip off low leaves, which were swallowed whole, since the teeth were of no use for chewing. Its front feet had five toes.

Triceratops was a four-legged, plant-eating dinosaurs, weighing several tons, that lived during the Cretaceous period. Triceratops had three horns: one centrally located just above the nostrils and a pair that projected from the forehead. The skull was quite large in proportion to the rest of the body. A bony frill at the back of the skull protected the neck and anchored powerful jaw and neck muscles. With head lowered and horns pointing forward, all backed up by its enormous bulk, Triceratops must have been a tough challenge to predators such as Tyrannosaurus rex.

Skull Story

Triceratop' heavy skull can tell us a lot about its way of life. Its jaw was built to tackle very tough and fibrous plants. It snipped them off with its narrow, hooked beak and sliced them up with its sharp, scissor-like teeth. The jaws were powered by huge muscles that extended up into the frill. The frill probably acted as an anchor for the jaw muscles, and also protected the neck.

Triceratops used its sharp horns mainly for defense against predators, but it also used them in one-to-one combat. The male Triceratops would lock horns with a member of its own kind and head-wrestle, much as deer, antelope, and sheep do today.

Pachycephalosaurs were small bipeds with thickened dome-shaped skulls, flattened bodies, and bony tails.

Thick Neck: They had bony lumps on their thick upper neck. The vertebrae in their necks were thick and strong and fitted firmly, with inimal twisting. The muscles were big and powerful, too. This adds support to the theory that the head could absorb tremendous jarring shocks.

Bonehead: Many, such as the Pachycephalosaurus ("thick-headed reptiles"), had a skull capped by a rounded dome of solid bone being 9.7 inches (25 cm) thick. They may have, then, charged at enemies or perhaps each other to take charge of the herd, or a female at mating season, as rams do today.

Stiff-backed, Rod-rigid: Strips of muscles anchored to the vertebrae ran from the base of the neck all the way down the back, to keep the spine straight and stiff. Small bony rods linked the vertebrae of the backbone and helped stiffen it in the back and hip area.

Knees Up: The iliotibialis muscle joined the ilial part of the hipbone (ischium) to the shin, or tibia, to raise the knee.

Third lever: They had long, slim bones. Like the thighs and shin, the foot was a lever to increase the running efeciency.

Pachycephalosaurus was a plant-eating, birdlike, beaked dinosaur that lived about 97 million to about 65 million years ago. This two-legged dinosaur was about 8 m (about 27 ft) long; its head was 25 cm (10 in) thick with a crown of spikes and bumps and little space for its small brain. Its body was designed like a battering ram, with a spine that locked rigidly to withstand impacts. Pachycephalosaurus's thick skull may have allowed it to butt heads with others of its kind without injuring its brain. It may have fought by ramming the soft bellies of other dinosaurs.